What Is a 51% Attack in a Proof of Work System?

A 51% attack is a potential attack on a blockchain where a single entity or group controls more than 50% of the network's mining hash rate. This control would allow them to prevent new transactions from gaining confirmations, and they could also reverse transactions that they sent while in control.

This would enable them to double-spend coins. However, they would not be able to create new coins or alter old blocks.

The immense cost of acquiring and operating the necessary hardware makes a 51% attack on a major blockchain like Bitcoin highly impractical.